1. Field of the Invention
The present invention relates generally to a wafer pattern imaging apparatus, and more particularly to a wafer pattern imaging apparatus that is applied to align a wafer in a wafer dicing apparatus, or the like.
2. Description of Related Art
Conventionally, a wafer is diced into a number of chips as described below. First, a wafer mounter aligns the wafer with a frame for transporting the wafer, and adheres the wafer to an adhesive tape (which will hereafter be referred to as a wafer sheet) that is adhered to the frame. The wafer as well as the frame is transferred to a wafer table, on which the wafer is diced. The reverse of the wafer, on which no pattern is formed, is adhered to the wafer sheet, and the wafer is placed with its obverse (which will hereafter be referred to as a pattern surface) up on the wafer table.
Then, the wafer is aligned on the wafer table. A pattern matching method is already known for aligning the wafer. In the pattern matching method, the pattern surface of the wafer is imaged at a plurality of positions thereof, and the obtained images are compared with a reference pattern stored in a computer to find a position of the reference pattern on the wafer. The position of the wafer is determined from the position of the reference pattern. The position of the wafer table is adjusted in accordance with the determined position of the wafer so as to properly align the wafer.
The wafer, which is adhered to the wafer sheet, is diced into a number of chips with a blade such as a diamond cutter. After the dicing, swarf cut from the wafer in the dicing is washed away by pure water, etc. Then, the chips are detached from the wafer sheet, and the dicing is completed.
In the dicing process, however, contaminations such as the swarf are adhered to the pattern surface of the wafer. Conventionally, the contaminations are washed away by pure water, etc. after the wafer is diced as stated above, or a cutting solution is supplied to the wafer during the dicing in such a manner as to prevent the contaminations from being adhered to the wafer. In these methods, it is impossible to eliminate the contaminantions from the pattern surface of the wafer.
Moreover, the cutting solution, etc. must not contact with the pattern surfaces of some kinds of wafers. In this case, it requires a great deal of trouble to prevent the contaminations from being adhered to the wafer. For example, the pattern surface of the wafer must be covered with a protection film, or the like.
To eliminate the above-mentioned disadvantages, the pattern surface of the wafer may be adhered to the wafer sheet to protect the pattern surface. This prevents the cutting solution, etc. from contacting with the pattern surface of the wafer and prevents the contaminations from being adhered to the pattern surface of the wafer.
In this case, however, the wafer is placed with its pattern surface down on the wafer table, and thus, an ordinary camera cannot image the pattern surface of the wafer from above the wafer table. Therefore, it is impossible to align the wafer according to the images of the pattern surface in the pattern matching method.